U.S. patent application number 10/432682 was filed with the patent office on 2005-08-18 for homeotropic alignment layer.
Invention is credited to Coates, David, Goulding, Mark John, Heckmeier, Michael, Kitson, Stephen Christopher, Klassen-Memmer, Melanie, Lussem, Georg, Parri, Owain Llyr, Tarumi, Kazuaki, Verrall, Mark.
Application Number | 20050179003 10/432682 |
Document ID | / |
Family ID | 8170423 |
Filed Date | 2005-08-18 |
United States Patent
Application |
20050179003 |
Kind Code |
A1 |
Heckmeier, Michael ; et
al. |
August 18, 2005 |
Homeotropic alignment layer
Abstract
The invention relates to an alignment layer comprising a
polymerized liquid crystal material with homeotrophic orientation,
to methods of its preparation, to polymerizable liquid crystal
compositions and liquid crystal polymers used for the preparation
of the alignment layer, to liquid crystal devices comprising the
alignment layer, and to a method of controlling the electrooptical
steepness of a liquid crystal display comprising at least one
alignment layer by varying the surface anchoring energy of the
alignment layer.
Inventors: |
Heckmeier, Michael;
(Hemsbach, DE) ; Klassen-Memmer, Melanie;
(Heuchelheim, DE) ; Lussem, Georg; (Ober-Ramstadt,
DE) ; Tarumi, Kazuaki; (Seeheim-Jugenheim, DE)
; Coates, David; (Wimborne, GB) ; Parri, Owain
Llyr; (Poole, GB) ; Verrall, Mark; (Salisbury
Wiltshire, GB) ; Goulding, Mark John; (Ringwood,
GB) ; Kitson, Stephen Christopher; (South
Gloucestershire, GB) |
Correspondence
Address: |
MILLEN, WHITE, ZELANO & BRANIGAN, P.C.
2200 CLARENDON BLVD.
SUITE 1400
ARLINGTON
VA
22201
US
|
Family ID: |
8170423 |
Appl. No.: |
10/432682 |
Filed: |
December 2, 2003 |
PCT Filed: |
November 22, 2001 |
PCT NO: |
PCT/EP01/13584 |
Current U.S.
Class: |
252/299.01 |
Current CPC
Class: |
G02F 1/133726 20210101;
G02F 1/133711 20130101; C09K 2323/03 20200801; G02F 1/1391
20130101; G02F 1/133742 20210101; G02F 1/1393 20130101; C09K
2323/02 20200801; C09K 2323/00 20200801; G02F 1/133715
20210101 |
Class at
Publication: |
252/299.01 |
International
Class: |
C09K 019/52 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 23, 2000 |
EP |
00125235.2 |
Claims
1. Alignment layer comprising a polymerized liquid crystal material
with homeotropic orientation.
2. Alignment layer according to claim 1, characterized in that the
tilt angle in the polymerized liquid crystal material is from 0 to
5.degree. relative to the normal of the layer.
3. Alignment layer according to claim 1, characterized in that the
liquid crystal material is polymerized in its nematic phase.
4. Alignment layer according to claim 1, characterized in that the
liquid crystal material is polymerized in its smectic phase.
5. Alignment layer according to claim 1, characterized in that the
liquid crystal material is polymerized in its smectic A phase.
6. Alignment layer according to claim 1, characterized in that the
polymerized liquid crystal material forms a three-dimensional
network.
7. Alignment layer according to claim 1, characterized in that it
has a high surface anchoring energy.
8. Alignment layer according to claim 1, characterized in that the
tilt anchoring parameter .pi. is from 0 to 0.6.
9. Process of preparing an alignment layer according to claim 1 by
applying a layer of a polymerizable mesogenic or liquid crystal
material onto a substrate, aligning the material into homeotropic
orientation, polymerizing the material and optionally removing the
polymerized film from the substrate.
10. Polymerizable liquid crystal material with a nematic or smectic
phase for use in the preparation of an alignment layer according to
claim 1.
11. Polymerizable liquid crystal material according to claim 10,
characterized in that it exhibits a smectic A phase.
12. Polymerizable liquid crystal material according to claim 10,
characterized in that it comprises one or more polymerizable
mesogenic compounds of formula I P-Sp-X-MG-R I wherein P is a
polymerizable group, Sp is a spacer group or a single bond, X
--O--, --S--, --CO--, --COO--, --OCO--, --O--COO--,
--CO--NR.sup.0--, --NR.sup.0--CO--, --OCH.sub.2--, --CH.sub.2O--,
--SCH.sub.2--, --CH.sub.2S--, --CF.sub.2O--, --OCF.sub.2--,
--CF.sub.2S--, --SCF.sub.2--, --CH.sub.2CH.sub.2--,
--CF.sub.2CH.sub.2--, --CH.sub.2CF.sub.2--, --CF.sub.2CF.sub.2--,
--CH.dbd.N--, --N.dbd.CH--, --N.dbd.N--, --CH.dbd.CH--,
--CF.dbd.CH--, --CH.dbd.CF--, --CF.dbd.CF--, --C.ident.C--,
--CH.dbd.CH--COO--, --OCO--CH.dbd.CH-- or a single bond, MG is a
mesogenic group, R is H, F, Cl, Br, I, CN, SCN, SF.sub.5H,
NO.sub.2, or a straight-chain or branched alkyl group with 1 to 20
C-atoms, which may be unsubstituted, mono- or poly-substituted by
F, Cl, Br, I or CN, it being also possible for one or more
non-adjacent CH.sub.2 groups to be replaced, in each case
independently from one another, by --O--, --S--, --NH--,
--NR.sup.0--, --SiR.sup.0R.sup.00--, --CO--, --COO--, --OCO--,
--OCO--O--, --S--CO--, --CO--S--, --CH.dbd.CH-- or --C.ident.C-- in
such a manner that O and/or S atoms are not linked directly to one
another, or denotes P-Sp-X--, and R.sup.0 and R.sup.00 are
independently of each other H or alkyl with 1 to 12 C-atoms.
13. Polymerizable liquid crystal material according to claim 10,
characterized in that it comprises a) 25 to 80% of one or more
monoreactive polymerizable mesogenic compounds having an unpolar
terminal group, b) 5 to 40% of one or more monoreactive
polymerizable mesogenic compounds having a polar terminal group, c)
0 to 65% of one or more polymerizable mesogenic compounds having
two or more polymerizable groups, d) 0.01 to 5% of a
photoinitiator.
14. Polymerizable liquid crystal material according to claim 10,
characterized in that it comprises 40 to 60% by weight of one or
more compounds of formula I-1, 15 to 25% by weight of one or more
compounds of formula I-2 and 15 to 60% by weight of one or more
compounds of formula I-3 13wherein W is H or CH.sub.3, n is an
integer from 3 to 6, Z.sup.1 and Z.sup.2 are each independently
--COO-- or --OCO--, X.sup.1 and X.sup.2 are each independently H or
CH.sub.3, and R.sup.1 is alkyl or alkoxy with 1 to 20 C atoms.
15. Use of an alignment layer according to claim 1 for generating
homeotropic alignment in a liquid crystal medium.
16. Liquid crystal device comprising at least one alignment layer
according to claim 1.
17. Liquid crystal device comprising at least one alignment layer
according to claim 1 that is in contact with a liquid crystal
medium and induces homeotropic alignment in the liquid crystal
medium in the region of contact.
18. Liquid crystal device according to claim 16, characterized in
that it comprises a nematic, smectic or cholesteric liquid crystal
medium.
19. Liquid crystal device according to claim 16, characterized in
that it is a bistable or multistable liquid crystal display
device.
20. Liquid crystal device according to claim 16, characterized in
that it is a display device of the VA (vertically aligned), VAN
(vertically aligned nematic), VAC (vertically aligned cholesteric),
ECB (electrically controlled birefringence), DAP (deformation of
aligned phases), CSH (colour super homeotropic), hybrid alignment,
HAN (hybrid aligned nematic), SSCT (surface stabilized cholesteric
texture), PSCT (polymer stabilized cholesteric texture),
flexoelectric or ULH (uniformly lying helix) mode, which can be of
the transmissive, reflective or transflective type.
21. Liquid crystal display comprising a liquid crystal cell
comprising a liquid crystal medium provided between a first and a
second electrode, at least one of which is light-transmissive, and
wherein the liquid crystal molecules in said medium are oriented
homeotropically when no external field is applied, and an alignment
layer according to claim 1, which is provided on the inner surface
of at least one of said first and the second electrode such that it
is directly in contact with the liquid crystal medium and induces
homeotropic edge alignment in the liquid crystal medium.
22. Use of an alignment layer according to claim 1 as substrate, or
as alignment or auxiliary layer applied on a substrate, in the
preparation of anisotropic or liquid crystal polymer films with
homeotropic structure from polymerisable LC materials, in order to
induce homeotropic orientation in the polymerisable LC
material.
23. Anisotropic or liquid crystal polymer film with homeotropic
structure, characterized in that it is prepared from a
polymerisable liquid crystal material on a homeotropic alignment
layer according to claim 1.
24. A method of controlling the electrooptical steepness of a
liquid crystal display of the VA mode comprising at least one
alignment layer according to claim 1, by varying the surface
anchoring energy of said alignment layer.
Description
FIELD OF THE INVENTION
[0001] The invention relates to an alignment layer comprising a
polymerized liquid crystal material with homeotropic orientation,
to methods of its preparation, to polymerizable liquid crystal
compositions and liquid crystal polymers used for the preparation
of the alignment layer, to liquid crystal devices comprising the
alignment layer, and to a method of controlling the electrooptical
steepness of a liquid crystal display comprising at least one
alignment layer by varying the surface anchoring energy of the
alignment layer.
BACKGROUND AND PRIOR ART
[0002] In liquid crystal displays (LCDs) it is usually required to
control the alignment of the liquid crystal medium at the inner
surface of the substrates forming the display cell. For example,
parallel or tilted orientation of the liquid crystal molecules
relative to the plane of the substrate is achieved by applying
rubbed polyimide alignment layers to the substrate surfaces.
Another common method to induce uniform alignment is for example
the oblique evaporation of inorganic materials like silicon-oxide
(SiO.sub.x) onto the substrate surfaces.
[0003] Reviews of conventional alignment techniques are given for
example by I. Sage in "Thermotropic Liquid Crystals", edited by G.
W. Gray, John Wiley & Sons, 1987, pages 75-77, and by T. Uchida
and H. Seki in "Liquid Crystals--Applications and Uses Vol. 3",
edited by B. Bahadur, World Scientific Publishing, Singapore 1992,
pages 1-63. A review of alignment materials and techniques is given
by J. Cognard, Mol. Cryst. Liq. Cryst. 78, Supplement 1 (1981),
pages 1-77.
[0004] Many applications, like for example LCDs of the VA (vertical
aligned) or SSCT (surface stabilized cholesteric texture) mode,
require vertical or so-called homeotropic alignment of the liquid
crystal medium, wherein the liquid crystal molecules are oriented
with their long molecular axis substantially perpendicular to the
plane of the substrate. In prior art, the following techniques have
been suggested to achieve homeotropic alignment:
[0005] 1. Use materials that intrinsically have very low surface
energies, for example fluorinated polymers such as PTFE. In this
case the energy of the system is minimized by having the LC
molecules in contact with each other rather than with the surface.
This leads to a homeotropic alignment, but with rather a weak
anchoring energy.
[0006] 2. Coat the substrate surface with a surfactant that "seeds"
the required alignment. For homeotropic alignment one can achieve
this by using a layer of hydrocarbon chains tethered at one end to
the surface. Just from steric considerations one expects that if
the surface coverage is sufficient these chains will pack to be on
average normal to the surface. If the interaction between the
chains and the LC molecules is sufficiently strong then this
alignment should seed a homeotropic alignment in the LC. This is
the conventional approach to achieving homeotropic alignment and is
the basis of most organometallic chrome complexes and of lecithin
which are commmonly used in the research lab. This approach does
generate strong homeotropic alignment, but it depends on getting a
very uniform, very thin (ideally a monolayer) coating of the
material. This is often difficult to achieve. The stability of
these materials is often not ideal, and cells do sometimes exhibit
ageing presumably because the alignment layers become detached from
the surface and dissolve into the LC. In addition these materials
are often ionic and so result in an unwanted increase in the
conductivity of the LC.
[0007] 3. Coat the substrate surface with a polymer that induces
homeotropic alignment, for example a suitably modified polyimide
material. A disadvantage of these materials is that they require a
high temperature (typically about 180.degree. C.) bake to cure
them. If a plastic substrate is being used this may not be
desirable.
[0008] 4. Inorganic oxides e.g. SiO can give homeotropic alignment
when deposited onto the surface at a controlled angle. The
disadvantage of this approach is that the deposition can be
difficult to control over large areas and requires vacuum
deposition.
[0009] It has also been suggested in prior art to use liquid
crystal polymer layers for inducing planar or tilted alignment in a
liquid crystal display. U.S. Pat. No. 5,262,882 describes an
orientation layer for inducing planar orientation in a liquid
crystal display, consisting of a polymer network in which a low
molar mass liquid crystal material is dispersed. U.S. Pat. No.
5,155,160 discloses a liquid crystalline auxiliary layer for
inducing tilted orientation in a liquid crystal display cell, which
is formed from an anisotropic gel composition comprising a
mesogenic diacrylate and a low molar mass liquid crystal mixture.
JP 2000-212310, WO 00/46634 and WO 00/46635 disclose an alignment
layer for inducing a preferred pretilt in a liquid crystal medium,
which is obtained by photoalignment of a photopolymer or a
photopolymer/monomer mixture by photoradiation at an oblique angle
or by photoradiation with circularly polarized light.
[0010] It was an aim of the present invention to provide an
alignment layer that induces improved vertical or homeotropic
alignment in a liquid crystal medium, and does not show the
drawbacks of alignment layers of prior art as described above.
[0011] The inventors of the present invention have found that the
above drawbacks can be overcome, and satisfactory homeotropic
alignment of a liquid crystal medium can be achieved by using an
alignment layer of polymerized liquid crystal material comprising
rod-shaped molecules with homeotropic orientation, in particular a
layer of homeotropic nematic or homeotropic smectic A liquid
crystal polymerized material.
[0012] In particular it was found that an alignment layer of
homeotropic liquid crystal polymerized material exhibits a
particularly high surface anchoring energy and yields strong
homeotropic alignment in a liquid crystal medium.
[0013] Another aspect of the invention relates to the influence of
the alignment force on the steepness of the electrooptical curve of
an LCD, in particular of a VA mode LCD. Thus, the inventors have
surprisingly found that there is a correlation between the surface
anchoring energy of vertically aligned liquid crystals, expressed
by the tilt anchoring parameter, and the corresponding steepness of
the electrooptical curve in LC displays, especially in VA mode
displays. In detail, the steepness of the electrooptical curve was
found to increase with decreasing anchoring energy.
[0014] Based on this finding it is possible to control the
steepness of the electrooptical curve of an LCD, in particular of a
VA mode display, by using alignment layers of varying anchoring
strength. In practical applications of LCDs it is often desired to
reduce the steepness of the electrooptical curve to allow for
better grey level differentiation. This can be achieved by using
inventive alignment layers which exhibit strong anchoring energy
and thus lead to reduced steepness.
SUMMARY OF THE INVENTION
[0015] One aspect of the present invention is an alignment layer
comprising a polymerized liquid crystal material with homeotropic
orientation.
[0016] Another aspect of the present invention is an alignment
layer as described above or below, comprising a polymerized liquid
crystal material that is polymerized in its nematic or smectic, in
particular in its smectic A phase.
[0017] Another aspect of the present invention is an alignment
layer as described above or below, comprising a polymerized liquid
crystal material with homeotropic orientation having a high surface
anchoring energy.
[0018] Another aspect of the present invention is a process of
preparing an alignment layer as described above or below, by
applying a layer of a polymerizable mesogenic or liquid crystal
material onto a substrate, aligning the material into homeotropic
orientation, polymerizing the material and optionally removing the
polymerized film from the substrate.
[0019] Another aspect of the present invention is a polymerizable
liquid crystal material with a nematic or smectic phase, in
particular a smectic A phase, that can be used for the preparation
of an alignment layer with homeotropic orientation as described
above or below.
[0020] Another aspect of the present invention is the use of an
alignment layer as described above or below for generating
homeotropic alignment in a liquid crystal medium.
[0021] Another aspect of the present invention is a liquid crystal
device, in particular a liquid crystal display, comprising an
alignment layer as described above and below.
[0022] Another aspect of the present invention is a liquid crystal
device comprising an alignment layer as described above and below
that is in contact with a liquid crystal medium and induces
homeotropic alignment in the liquid crystal medium in the regions
of contact.
[0023] Another aspect of the present invention is a bistable or
multistable liquid crystal device, in particular a bistable or
multistable display comprising a nematic or cholesteric liquid
crystal medium, wherein the liquid crystal medium can adopt at
least two stable configurations, comprising an alignment layer as
described above and below.
[0024] Another aspect of the present invention is a liquid crystal
display of the VA (vertically aligned), VAN (vertically aligned
nematic) or VAC (vertically aligned cholesteric), ECB (electrically
controlled birefringence), DAP (deformation of aligned phases), CSH
(colour super homeotropic), hybrid alignment, HAN (hybrid aligned
nematic), SSCT (surface stabilized cholesteric texture), PSCT
(polymer stabilized cholesteric texture), flexoelectric or ULH
(uniformly lying helix) mode, which can be of the transmissive,
reflective or transflective type, comprising an alignment layer as
described above and below.
[0025] Another aspect of the present invention is the use of an
alignment layer as described above or below as substrate, or as
alignment or auxiliary layer applied on a substrate, in the
preparation of anisotropic or liquid crystal polymer films with
homeotropic structure from polymerisable LC materials, in order to
induce homeotropic orientation in the polymerisable LC
material.
[0026] Another aspect of the present invention is an anisotropic or
liquid crystal polymer film with homeotropic structure which is
prepared from a polymerisable liquid crystal material on a
homeotropic alignment layer as described above and below.
[0027] Another aspect of the present invention is a method of
controlling the electrooptical steepness of a liquid crystal
display, in particular a display of the VA mode, comprising at
least one alignment layer, by varying the surface anchoring energy
of the alignment layer.
[0028] Definition of Terms
[0029] In connection with a liquid crystal medium provided between
two substrates or a liquid crystal polymer film as described in the
present application, the following definitions of terms as used
throughout this application are given.
[0030] The terms `vertical or homeotropic structure`, `vertical or
homeotropic orientation` and `vertical or homeotropic alignment`
mean that the optical axis of the film or medium is substantially
perpendicular to the film plane or the substrate plane,
respectively, i.e. substantially parallel to the film or substrate
normal. This definition also includes films and media wherein the
optical axis is slightly tilted at an angle of up to 2 to 5 degrees
relative to the film normal or substrate normal.
[0031] For sake of simplicity, a film with a homeotropic
orientation is hereinafter being shortly referred to as
`homeotropic film`.
[0032] The term `film` as used in this application includes
self-supporting, i.e. free-standing, films that show more or less
pronounced mechanical stability and flexibility, as well as
coatings or layers on a supporting substrate or between two
substrates.
[0033] The term `liquid crystal or mesogenic material` or `liquid
crystal or mesogenic compound` should denote materials or compounds
comprising one or more rod-shaped, board-shaped or disk-shaped
mesogenic groups, i.e. groups with the ability to induce liquid
crystal phase behaviour. The compounds or materials comprising
mesogenic groups do not necessarily have to exhibit a liquid
crystal phase themselves. It is also possible that they show liquid
crystal phase behaviour only in mixtures with other compounds, or
when the mesogenic compounds or materials, or the mixtures thereof,
are polymerized.
[0034] For sake of simplicity, the term `liquid crystal material`
is used hereinafter for both liquid crystal materials and mesogenic
materials, and the term `mesogen` is used for the mesogenic groups
of the material.
[0035] In a liquid crystal display according to the present
invention comprising a liquid crystal medium and an alignment
layer, the surface anchoring strength of the liquid crystal
molecules at the surface of the alignment layer is expressed by the
tilt anchoring parameter .pi..
[0036] A value of .pi.32 0 means that the liquid crystal molecules
are strongly anchored. A value of .pi.=1 means that the liquid
crystal molecules are freely rotating, without being significantly
anchored. The values of .pi. are correlated to experimental data
according to Mada et al., Jap. Journ. Appl. Phys. 38, L1118-L1120
(1999). Thus, a value of .pi.=1 (not anchored) corresponds to a
surface anchoring energy of about 10.sup.-10 J/m.sup.2, and a value
of .pi.=0 (strongly anchored) corresponds to a surface anchoring
energy of about 10.sup.-5 J/m.sup.2.
[0037] The surface energy can be determined for example by contact
angle measurements using standard equipment and methods that are
known to those skilled in the art. The determination of the surface
anchoring energy is also described e.g. in the following
references:
[0038] Y. Marinov, N. Shonova, C. Versace, A. G. Petrov,
"Flexoelectric spectroscopy measurements of surface dissipation of
energy and surface viscosity of weakly anchored homeotropic
nematics" Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A (1999),
329, 1145-1150.
[0039] V. Sergan, G. Durand, "Anchoring anisotropy of a nematic
liquid crystal on a bistable SiO evaporated surface", Liq. Cryst.
(1995), 18(1), 1714,
[0040] M. Vilfan, M. Copic, "Comparison of dynamic and static
measurements of surface anchoring energy in nematic liquid
crystals", Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A (2000),
351, 419-426,
[0041] J. G. Fonseca, Y. Galerne, "Local measurement of the
zenithal anchoring strength", Phys. Rev. E: Stat. Phys., Plasmas,
Fluids, Relat. Interdiscip. Top. (2000), 61(2), 1550-1558,
[0042] T. Akahane, H. Kaneko, M. Kimura, "Novel method of measuring
surface torsional anchoring strength of nematic liquid crystals",
Jpn. J. Appl. Phys., Part 1 (1996), 35(8), 4434-4437,
[0043] JP-A-06-265840, "Method for measuring boundary anchoring
strength of liquid crystal display device",
[0044] A. Sugimura, T. Miyamoto, M. Tsuji, M. Kuze, Appl. Phys.
Lett. (1998), 72(3), 329-331,
[0045] F. Yang, J. R. Sambles, J. Appl. Phys. (2000), 87(6),
2726-2735.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] FIGS. 1.1 and 1.2 show the electrooptic curve (transmission
versus applied voltage) and the full steepness curve (numerical
gradient of the electrooptical curve), respectively, of a VA cell
according to example 3 of the present invention comprising an
alignment layer with a tilt anchoring parameter of 0.
[0047] FIGS. 2.1 and 2.2 show the electrooptic curve and full
steepness curve of a VA cell according to example 3 of the present
invention comprising an alignment layer with a tilt anchoring
parameter of 0.4.
[0048] FIGS. 3.1 and 3.2 show the electrooptic curve and full
steepness curve of a VA cell according to example 3 of the present
invention comprising an alignment layer with a tilt anchoring
parameter of 1.
[0049] FIGS. 4.1 and 4.2 show the electrooptic curve and full
steepness curve of a VA cell according to example 3 of the present
invention comprising an alignment layer with a tilt anchoring
parameter of (from left to right) 1, 0.8, 0.6, 0.4 and 0,
respectively.
DETAILED DESCRIPTION OF THE INVENTION
[0050] Preferred embodiments of the invention relate to alignment
layers wherein
[0051] the tilt angle of the polymerized liquid crystal material is
from 0 to 5.degree. relative to the normal of the layer,
[0052] the tilt angle of the polymerized liquid crystal material is
from 0 to 2.degree., preferably from 0 to 1.degree., very
preferably 0.degree., relative to the normal of the layer,
[0053] the polymerizable liquid crystal material exhibits a nematic
phase,
[0054] the polymerizable liquid crystal material exhibits a smectic
phase,
[0055] the polymerizable liquid crystal material exhibits a smectic
A phase,
[0056] the polymerized liquid crystal material exhibits a nematic
phase,
[0057] the polymerized liquid crystal material exhibits a smectic
phase,
[0058] the polymerized liquid crystal material exhibits a smectic A
phase,
[0059] the polymerized liquid crystal material forms a
3-dimensional network,
[0060] the polymerizable liquid crystal material comprises less
than 50%, preferably less than 20%, very preferably less than 10%,
in particular less than 5% of non-polymerizable compounds,
[0061] the polymerized liquid crystal material comprises less than
50%, preferably less than 20%, very preferably less than 10%, in
particular less than 5% of unpolymerized material,
[0062] the tilt anchoring parameter .pi. is from 0 to 0.6, very
preferably from 0 to 0.4.
[0063] A homeotropic alignment layer of the present invention can
be prepared by coating a substrate with a thin layer of a
polymerizable LC material, which is then polymerized to form a
solid layer. When coated onto a substrate with an open surface in
contact with air or an inert gas like for example nitrogen, the
polymerizable LC material prior to polymerization will tend to
align with the LC director normal to the LC/air boundary and hence
normal to the substrate. Homeotropic alignment of the coated
polymerizable LC material can also be induced or improved by known
methods like surface treatment or rubbing of the substrate. The
preparation of polymerized liquid crystal films with homeotropic
orientation is also described in WO 98/00475, the entire disclosure
of which, and of all patents issuing thereof, is incorporated into
this application by way of reference.
[0064] This alignment is then fixed by polymerizing the coated LC
material, e.g. by exposure to heat or actinic radiation. A suitable
method is photopolymerization by exposure to UV light. When placed
in contact with an LC material, for example with a nematic LC
material, the homeotropic alignment of the polymerized LC alignment
material tends to seed the alignment of the nematic LC material so
that it too tends to align normal to the substrate surface, that is
homeotropic alignment.
[0065] The polymerizable LC alignment material of the present
invention has the advantages that it exhibits a particularly high
surface anchoring energy and yields strong homeotropic alignment in
a liquid crystal medium. Moreover, it is relatively easy to apply
to a substrate, does not require a high temperature bake, and is
more stable than the surfactants that are normally used in prior
art. In addition there is the potential to modify the chemical
nature of the material to optimize its interaction with the nematic
LC material and so tune the strength of the anchoring energy.
[0066] Alignment layers obtained from a liquid crystal material,
preferably with rod-shaped mesogens, that is polymerized in the
smectic phase, preferably in the smectic A phase, were found to
exhibit a high tilt anchoring energy and are particularly
preferred.
[0067] The polymerizable liquid crystal material preferably
comprises one or more polymerizable compounds having at least one
polymerizable group.
[0068] The polymerizable material may also comprise polymerizable
mesogenic compounds having two or more polymerizable functional
groups (herein also referred to as di-/multireactive or
di-multifunctional compounds). Upon polymerization of such a
mixture a three-dimensional polymer network is formed. An alignment
layer comprising such a network is self-supporting and shows a high
mechanical and thermal stability and low temperature dependence of
the physical and optical properties. An alignment layer comprising
a linear, i.e. non-crosslinked polymer or a polymer with low
crosslink density on the other hand shows usually better adhesion
to the substrate of the liquid crystal cell.
[0069] By varying the concentration of the multifunctional
compounds the crosslink density of the polymer film and thereby its
physical and chemical properties such as the glass transition
temperature, which is also important for the temperature dependence
of the optical properties of the optical retardation film, the
thermal and mechanical stability or the solvent resistance can be
tuned easily.
[0070] Another aspect of the invention relates to the influence of
the alignment force on the steepness of the electrooptical curve of
an LCD, in particular of a VA mode LCD. The inventors have found
that there is a correlation between the surface anchoring energy of
vertically aligned liquid crystals, expressed by the tilt anchoring
parameter .pi., and the corresponding steepness of the
electrooptical curve in LC displays, especially in VA mode
displays. The steepness of the electrooptical curve was found to
increase with decreasing anchoring energy.
[0071] By using an alignment layer of the present invention with
high anchoring energy, it is possible to reduce the steepness of
the electrooptical curve of an LCD, in particular of a VA mode
display.
[0072] Furthermore, by varying the anchoring energy of the
alignment layer, it is possible to control the steepness of the
electrooptical curve of the display and adapt it to the desired
use. The anchoring energy of an alignment layer of the invention
can for example be controlled by varying the parameters of the
polymerizable material and the polymerized alignment layer. For the
polymerizable material such parameters are for example the chemical
composition or the liquid crystalline phase behaviour. For example
a polymerizable material with a smectic phase yields an alignment
layer with a particular high surface anchoring energy. For the
polymerized alignment layer such parameters are for example the
polymerization conditions, the degree of polymerization, the layer
thickness or the molecular weight, crosslink density or chain
length of the polymer. The molecular weight or chain length of the
polymer can for example be reduced by using chain transfer
agents.
[0073] Especially preferably the polymerizable liqud crystal
material comprises one or more compounds selected of formula I
P-Sp-X-MG-R I
[0074] wherein
[0075] P is a polymerizable group,
[0076] Sp is a spacer group or a single bond,
[0077] X is --O--, --S--, --CO--, --COO--, --OCO--, --O--COO--,
--CO--NR.sup.0--, --NR.sup.0--CO-- --OCH.sub.2--, --CH.sub.2O--,
--SCH.sub.2--, --CH.sub.2S--, --CF.sub.2O--, --OCF.sub.2--,
--CF.sub.2S--, --SCF.sub.2--, --CH.sub.2CH.sub.2--,
--CF.sub.2CH.sub.2--, --CH.sub.2CF.sub.2--, --CF.sub.2CF.sub.2--,
--CH.dbd.N--, --N.dbd.CH--, --N.dbd.N--, --CH.dbd.CH--,
--CF.dbd.CH--, --CH.dbd.CF--, --CF.dbd.CF--, --C.ident.C--,
--CH.dbd.CH--COO--, --OCO--CH.dbd.CH-- or a single bond,
[0078] MG is a mesogenic group,
[0079] R is H, F, Cl, Br, I, CN, SCN, SF.sub.5H, NO.sub.2, or a
straight--cahin or branched alkyl group with 1 to 20 C-atoms, which
may be unsubstituted, mono- or poly-substituted by F, Cl, Br, I or
CN, it being also possible for one or more non-adjacent CH.sub.2
groups to be replaced, in each case independently from one another,
by --O--, --S--, --NH--, --NR.sup.0--, --SiR.sup.0R.sup.00--,
--CO--, --COO--, --OCO--, --OCO--O--, --S--CO--, --CO--S--,
--CH.dbd.CH-- or --C.ident.C-- in such a manner that O and/or S
atoms are not linked directly to one another, or denotes P-Sp-X--,
and
[0080] R.sup.0 and R.sup.00 are independently of each other H or
alkyl with 1 to 12 C-atoms.
[0081] In a preferred embodiment of the present invention MG is of
formula II
A.sup.1-(Z-A.sup.2).sub.m-- II
[0082] wherein
[0083] Z is in each case independently --O--, --S--, --CO--,
--COO--, --OCO--, --O--COO--, --CO--NR.sup.0--, --NR.sup.0--CO--,
--OCH.sub.2--, --CH.sub.2O--, --SCH.sub.2--, --CH.sub.2S--,
--CF.sub.2O--, --OCF.sub.2--, --CF.sub.2S--, --SCF.sub.2--,
--CH.sub.2CH.sub.2--, --CF.sub.2CH.sub.2--, --CH.sub.2CF.sub.2--,
--CF.sub.2CF.sub.2--, --CH.dbd.N--, --N.dbd.CH--, --N.dbd.N--,
--CH.dbd.CH--, --CF.dbd.CH--, --CH.dbd.CF--, --CF.dbd.CF--,
--C.ident.C--, --CH.dbd.CH--COO-- or --OCO--CH.dbd.CH--,
[0084] R.sup.0 has one of the meanings given in formula I,
[0085] A.sup.1 and A.sup.2 are each independently 1,4-phenylene in
which, in addition, one or more CH groups may be replaced by N,
1,4-cyclohexylene in which, in addition, one or two non-adjacent
CH.sub.2 groups may be replaced by O and/or S, 1,4-cyclohexenylene,
indane-2,5-diyl, 1,4-bicyclo (2,2,2)octylene, piperidine-1,4-diyl,
naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl, or
1,2,3,4-tetrahydronaphthalene-2,6-diyl, it being possible for all
these groups to be unsubstituted, mono- or polysubstituted with F,
Cl, OH, CN or NO.sub.2 or alkyl, alkoxy, alkylcarbonyl or
alkoxycarbonyl groups having 1 to 7 C atoms wherein one or more H
atoms may be substituted by F or Cl, and
[0086] m is 0, 1, 2 or 3.
[0087] Particularly preferred are compounds wherein the mesogenic
group A.sup.1-(Z-A.sup.2).sub.m incorporates two or three five- or
six-membered rings.
[0088] Further preferred are compounds wherein at least one radical
Z denotes --C.ident.C--. These compounds are especially suitable
for uses where highly birefringent materials are needed.
[0089] A smaller group of preferred mesogenic groups of formula II
is listed below. For reasons of simplicity, Phe in these groups is
1,4 phenylene, PheL is a 1,4-phenylene group which is substituted
by 1 to 4 groups L, with L being F, Cl, OH, CN, NO.sub.2 or an
optionally fluorinated alkyl, alkoxy, alkylcarbonyl or
alkoxycarbonyl group with 1 to 7 C atoms, and Cyc is
1,4-cyclohexylene. Z has one of the meanings of formula II. The
list is comprising the following subformulae as well as their
mirror images
1 -Phe-Z-Phe- II-1 -Phe-Z-Cyc- II-2 -Cyc-Z-Cyc- II-3 -PheL-Z-Phe-
II-4 -PheL-Z-Cyc- II-5 -PheL-Z-PheL- II-6 -Phe-Z-Phe-Z-Phe- II-7
-Phe-Z-Phe-Z-Cyc- II-8 -Phe-Z-Cyc-Z-Phe- II-9 -Cyc-Z-Phe-Z-Cyc-
II-10 -Phe-Z-Cyc-Z-Cyc- II-11 -Cyc-Z-Cyc-Z-Cyc- II-12
-Phe-Z-Phe-Z-PheL- II-13 -Phe-Z-PheL-Z-Phe- II-14
-PheL-Z-Phe-Z-Phe- II-15 -PheL-Z-Phe-Z-PheL- II-16
-PheL-Z-PheL-Z-Phe- II-17 -PheL-Z-PheL-Z-PheL- II-18
-Phe-Z-PheL-Z-Cyc- II-19 -Phe-Z-Cyc-Z-PheL- II-20
-Cyc-Z-Phe-Z-PheL- II-21 -PheL-Z-Cyc-Z-PheL- II-22
-PheL-Z-PheL-Z-Cyc- II-23 -PheL-Z-Cyc-Z-Cyc- II-24
-Cyc-Z-PheL-Z-Cyc- II-25
[0090] Particularly preferred are the subformulae II-1, II-2, II-4,
II-6, II-7, II-8, II-11, II-13, II-14, II-15 and II-16.
[0091] Preferably Z is --COO--, --OCO--, --CH.sub.2CH.sub.2--,
--C.ident.C-- or a single bond.
[0092] Very preferably the mesogenic group MG is selected from the
following formulae and their mirror images 12
[0093] wherein L has the meaning given above and r is 0, 1 or
2.
[0094] The group 3
[0095] in these preferred formulae is very preferably denoting
4
[0096] with L having each independently one of the meanings given
above. Particularly preferred are the subformulae IId, IIg, IIh,
IIi, IIk and IIo, in particular the subformulae IId and IIk.
[0097] L is preferably F, Cl, CN, OH, NO.sub.2, CH.sub.3,
C.sub.2H.sub.5, OCH.sub.3, OC.sub.2H.sub.5, COCH.sub.3,
COC.sub.2H.sub.5, COOCH.sub.3, COOC.sub.2H.sub.5, CF.sub.3,
OCF.sub.3, OCHF.sub.2, OC.sub.2F.sub.5, in particular F, Cl, CN,
CH.sub.3, C.sub.2H.sub.5, OCH.sub.3, COCH.sub.3, CF.sub.3 and
OCF.sub.3, most preferably F, Cl, CH.sub.3, OCH.sub.3 and
OCF.sub.3.
[0098] If R in formula I is an alkyl or alkoxy radical, i.e. where
the terminal CH.sub.2 group is replaced by --O--, this may be
straight-chain or branched.
[0099] It is preferably straight-chain, has 2, 3, 4, 5, 6, 7 or 8
carbon atoms and accordingly is preferably ethyl, propyl, butyl,
pentyl, hexyl, heptyl, octyl, ethoxy, propoxy, butoxy, pentoxy,
hexoxy, heptoxy, or octoxy, furthermore methyl, nonyl, decyl,
undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, nonoxy, decoxy,
undecoxy, dodecoxy, tridecoxy or tetradecoxy, for example.
[0100] Especially preferably R is straight chain alkyl or alkoxy
with 1 to 8 C atoms.
[0101] Oxaalkyl, i.e. where one CH.sub.2 group is replaced by
--O--, is preferably straight-chain 2-oxapropyl (=methoxymethyl),
2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3-, or
4-oxapentyl, 2-, 3-, 4-, or 5-oxahexyl, 2-, 3-, 4-, 5-, or
6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-,
7- or 8-oxanonyl or 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl, for
example.
[0102] Halogen is preferably F or Cl.
[0103] R in formula I can be a polar or an unpolar group. In case
of a polar group, R is selected from CN, NO.sub.2, halogen,
OCH.sub.3, OCN, SCN, COR.sup.1, COOR.sup.1 or a mono- oligo- or
polyfluorinated alkyl or alkoxy group with 1 to 4 C atoms. R.sup.1
is optionally fluorinated alkyl with 1 to 4, preferably 1 to 3 C
atoms. Especially preferably polar groups R are selected of F, Cl,
CN, NO.sub.2, OCH.sub.3, COCH.sub.3, COC.sub.2H.sub.5, COOCH.sub.3,
COOC.sub.2H.sub.5, CF.sub.3, C.sub.2F.sub.5, OCF.sub.3, OCHF.sub.2,
and OC.sub.2F.sub.5, in particular of F, Cl, CN, OCH.sub.3 and
OCF.sub.3. In case of an unpolar group, R is preferably alkyl with
up to 15 C atoms or alkoxy with 2 to 15 C atoms.
[0104] Compounds of formula I containing an achiral branched group
R may occasionally be of importance, for example, due to a
reduction in the tendency towards crystallization. Branched groups
of this type generally do not contain more than one chain branch.
Preferred achiral branched groups are isopropyl, isobutyl
(=methylpropyl), isopentyl (=3-methylbutyl), isopropoxy,
2-methyl-propoxy and 3-methylbutoxy.
[0105] Another preferred embodiment of the present invention
relates to compounds of formula I wherein R is denoting
P-Sp-X.sub.n--.
[0106] The polymerisable group P is preferably selected from
CH.sub.2.dbd.CW.sup.1--COO--, 5
[0107] CH.sub.2.dbd.CW.sup.2--O--, CH.sub.3--CH.dbd.CH--O--,
HO--CW.sup.2W.sup.3--, HS--CW.sup.2W.sup.3--, HW.sup.2N--,
HO--CW.sup.2W.sup.3--NH--, CH.sub.2.dbd.CW.sup.1--CO--NH--,
CH.sub.2.dbd.CH--(COO).sub.k1-Phe-(O).sub.k2--, Phe-CH.dbd.CH--,
HOOC--, OCN-- and W.sup.4W.sup.5W.sup.6Si--, with W.sup.1 being H,
Cl, CN, phenyl or alkyl with 1 to 5 C-atoms, in particular H,
C.sub.1 or CH.sub.3, W.sup.2 and W.sup.3 being independently of
each other H or alkyl with 1 to 5 C-atoms, in particular methyl,
ethyl or n-propyl, W.sup.4, W.sup.5 and W.sup.6 being independently
of each other Cl, oxaalkyl or oxacarbonylalkyl with 1 to 5 C-atoms,
Phe being 1,4-phenylene and k.sub.1 and k.sub.2 being independently
of each other 0 or 1.
[0108] P is particularly preferably an acrylate, methacrylate,
vinyl, vinyloxy, epoxy, styrene or propenyl ether group, in
particular an acrylate, methacrylate, vinyl or epoxy group.
[0109] As for the spacer group Sp all groups can be used that are
known for this purpose to those skilled in the art. The space group
Sp is preferably a straight chain or branched alkylene group having
1 to 20 C atoms, in particular 1 to 12 C atoms, in which, in
addition, one or more non-adjacent CH.sub.2 groups may be replaced
by --O--, --S--, --NR.sup.0--, --CO--, --O--CO--, --S--CO--,
--O--COO--, --CO--S--, --CO--O--, --CH(halogen)-,
--C(halogen).sub.2, --CH(CN)--, --CH(OH)--, --CD.sub.2-,
--CH.dbd.CH--, --CF.dbd.CF--, --CH.dbd.CF-- or --C.ident.C--, or a
siloxane group, and in which one or more H atoms may be replaced by
halogen, CN or OH.
[0110] Typical spacer groups are for example --(CH.sub.2).sub.o--,
--(CH.sub.2CH.sub.2O).sub.p--CH.sub.2CH.sub.2--,
--CH.sub.2CH.sub.2--S--C- H.sub.2CH.sub.2-- or
--CH.sub.2CH.sub.2--NH--CH.sub.2CH.sub.2--, or
--(SiR.sup.0R.sup.00--O).sub.q--, with o being an integer from 2 to
12, p being an integer from 1 to 3, q being an integer from 1 to 3,
and R.sup.0 and R.sup.00 having the meanings given above.
[0111] Preferred spacer groups are ethylene, propylene, butylene,
pentylene, hexylene, heptylene, octylene, nonylene, decylene,
undecylene, dodecylene, octadecylene, ethyleneoxyethylene,
methyleneoxybutylene, ethylene-thioethylene,
ethylene-N-methyliminoethylene, 1-methylalkylene, ethenylene,
propenylene and butenylene for example.
[0112] Especially preferred are compounds of formula I wherein Sp
is denoting alkylene or alkylene-oxy with 2 to 8 C atoms.
Straight-chain groups are especially preferred.
[0113] In the event that R is denoting P-Sp-X--, the two spacer
groups Sp in the compounds of formula I may be identical or
different.
[0114] Of the preferred compounds described above particularly
preferred are those wherein n is 1.
[0115] Further preferred are compounds with one or more groups
P-Sp-X-- wherein n is 0.
[0116] In case of multiple occurrence of the group P, Sp and X,
respectively, the multiply occurring groups may be identical or
different.
[0117] The compounds of formula I can be synthesized according to
or in analogy to methods which are known per se and which are
described in standard works of organic chemistry such as, for
example, Houben-Weyl, Methoden der organischen Chemie,
Thieme-Verlag, Stuttgart. Some specific methods of preparation can
be taken from the examples.
[0118] Examples of suitable polymerizable mesogenic compounds that
can be used as components of the polymerizable composition are
disclosed for example in WO 93/22397; EP 0,261,712; DE 195,04,224;
WO 95/22586 and WO 97/00600. The compounds disclosed in these
documents, however, are to be regarded merely as examples that
shall not limit the scope of this invention. Preferably the
polymerizable mixture comprises at least one polymerizable
mesogenic compound having one polymerizable functional group and at
least one polymerizable mesogenic compound having two or more
polymerizable functional groups.
[0119] Examples of especially useful polymerizable mesogenic
compounds are shown in the following list of compounds, which
should, however, be taken only as illustrative and is in no way
intended to restrict, but instead to explain the present invention:
67
[0120] In the above formulae, P has one of the meanings of formula
I and its preferred meanings as mentioned above, x and y are
identical or different integers from 1 to 12, A is 1,4-phenylene
that is unsubstituted or substituted in 2-, 3- and/or 5-position by
L.sup.1 or denotes 1,4-cyclohexylene, Z.sup.0 has one of the
meanings of Z.sup.1 in formula II, v is 0 or 1, Y is a polar group
as defined above, R.sup.0 is an unpolar alkyl or alkoxy group and
L.sup.1 and L.sup.2 are each independently H, F, Cl, CN, OH,
NO.sub.2 or an optionally halogenated alkyl, alkoxy or carbonyl
group with 1 to 4 C atoms, and the 1,4-phenylene rings in the above
formulae may also be substituted in 2-, 3- and/or 5-position by
L.sup.1.
[0121] The term unpolar group in this connection preferably denotes
an alkyl group with 1 or more, preferably 1 to 15 C atoms or an
alkoxy group with 2 or more, preferably 2 to 15 C atoms.
[0122] The orientation of the mesogenic material depends, inter
alia, on the film thickness, the type of substrate material, and
the composition of the polymerizable mesogenic material. It is
therefore possible, by changing these parameters, to control the
structure of the polymer film, in particular specific parameters
such as the tilt angle and its degree of variation.
[0123] Thus, for the preparation of a homeotropic film, it is
possible to improve the alignment by appropriate selection of the
ratio of monoreactive mesogenic compounds, i.e. compounds with one
polymerizable group, and direactive mesogenic compounds, i.e.
compounds with two polymerizable groups.
[0124] The amount of polymerizable compounds with two or more
polymerizable groups in the polymerizable material is preferably
from 5 to 25% by weight of the total mixture. In another preferred
embodiment, the polymerizable mixture contains no polymerizable
compounds with more than one polymerizable group.
[0125] Especially preferably the polymerizable liquid crystal
material comprises
[0126] a) 25 to 80%, in particular 30 to 70%, very preferably 40 to
60% of one or more monoreactive polymerizable mesogenic compounds,
preferably selected of formula I, having an unpolar terminal
group,
[0127] b) 5 to 40%, in particular 10 to 35%, very preferably 15 to
30% of one or more monoreactive polymerizable mesogenic compounds,
preferably selected of formula I, having a polar terminal
group,
[0128] c) 0 to 65%, in particular 2 to 45%, very preferably 5 to
25% of one or more polymerizable mesogenic compounds, preferably
selected of formula I, having two or more polymerizable groups,
[0129] d) 0.01 to 5% of a photoinitiator.
[0130] Very preferred are mixtures wherein the ratio of components
a:b is ranging from 5:2 to 3:2, and mixtures wherein the ratio of
monoreactive compounds to direactive compounds is ranging from 3:1
to 1:1. Further preferred are mixtures wherein the ratio of
components a:b:c is approximately 2:1:1.
[0131] Preferably the mixtures comprise 2 to 8, in particular 2 to
6, most preferably 2 to 4 compounds of components a and b, and 1 to
3 compounds of component c.
[0132] The mixtures may also comprise further components, like
stabilizers, inhibitors, chain transfer agents, dyes, surfactants
or non-mesogenic crosslinkers.
[0133] The compounds of component a) are preferably selected from
formula Id-Ii above. The compounds of component b) are preferably
selected from formulae Ia-Ic above. The compounds of component c)
are preferably selected from formula Ik and Im above.
[0134] The spacer groups in the compounds of formula Ia-Im and the
alkyl terminal groups in the compounds of formula Id-Ii are
preferably selected from propylene to hexylene. The alkyl chain
length (either spacer or terminal groups) of the components and
their concentration is optimised to provide a smectic phase at an
appropriate temperature above the temperature that will occur
during cure of the film. To increase smectic phase behaviour,
spacer groups or terminal alkyl groups higher than hexylene, like
for example heptylene, octylene, nonylene, decylene, undecylene or
dodecylene, are also suitable.
[0135] Especially preferred is a polymerizable mixture
comprising
[0136] a) 40 to 60% of one or more compounds of formula Ie, in
particular wherein v is 1 and A is 1,4-cyclohexylene,
[0137] b) 15 to 25% of one or more compounds of formula Ia, in
particular wherein v is 1 and Y is CN,
[0138] c) 5 to 30% of one or more compounds of formula Ik, in
particular wherein L.sup.1 is CH.sub.3 and L.sup.2 is H,
[0139] d) 0.01 to 5% of a photoinitiator.
[0140] The polymerizable mesogenic material is coated onto
substrate, aligned into a uniform orientation and polymerized
according to a process as described in WO 98/12584 or GB 2,315,072,
thereby permanently fixing the orientation of the polymerizable
mesogenic material.
[0141] As a substrate for example a glass or quartz sheet or a
plastic film or sheet can be used. Suitable plastic substrates are
for example films of polyethyleneterephthalate (PET),
polyvinylalcohol (PVA), polycarbonate (PC) or triacetylcellulose
(TAC).
[0142] The alignment layer of the present invention is preferably
prepared directly on the inner surface of the substrates or
electrodes forming the liquid crystal cell of a display. This is
achieved for example by coating and polymerizing the polymerizable
mesogenic material on the substrate, which is typically a glass
plate covered with a transparent layer of conductive material like
indium tin oxide (ITO) layer, for example by spin coating, and
polymerizing the coated and aligned material. The polymerizable
material can be applied directly on the ITO layer, however, the
substrate may also comprise an additional alignment layer above the
ITO layer, onto which the polymerizable material is coated. It is
also possible that the substrate comprises further layers, like
e.g. colour filters, protective or passviation layers or black
layers, on top or below the ITO layer.
[0143] It is also possible to put a second substrate on top of the
coated mixture prior to and/or during and/or after polymerization,
which can be removed after polymerization. When curing between two
substrates exposure to by actinic radiation, at least one substrate
has to be transmissive for the actinic radiation used for the
polymerization. Isotropic or birefringent substrates can be
used.
[0144] The polymerizable mesogenic material can also be dissolved
in a solvent, preferably in an organic solvent. The solution is
then coated onto the substrate, for example by spin-coating or
other known techniques, and the solvent is evaporated off before
polymerization. In most cases it is suitable to heat the mixture in
order to facilitate the evaporation of the solvent.
[0145] The preparation of polymerized liquid crystal films with
homeotropic orientation is also described in WO 98/00475, the
entire disclosure of which is incorporated into this application by
way of reference. Homeotropic alignment can be achieved e.g. by
means of an alignment layer coated on top of the substrate.
Suitable aligning agents used on glass substrates are for example
alkyltrichlorosilane, chrome complexes or lecithin, whereas for a
plastic substrate thin layers of lecithin, silica or high tilt
polyimide orientation films as aligning agents may be used. In a
preferred embodiment of the invention a silica coated plastic film
is used as a substrate. Furthermore, homeotropic alignment can be
achieved by using aluminium oxide films as described in GB
2,324,382.
[0146] Further suitable methods and agents to achieve homeotropic
alignment are described in T. Uchida and H. Seki in "Liquid
Crystals--Applications and Uses Vol. 3", edited by B. Bahadur,
World Scientific Publishing, Singapore 1992, pages 1-63, and in J.
Cognard, Mol. Cryst. Liq. Cryst. 78, Supplement 1 (1981), pages
1-77. Suitable materials are for example surface coupling agents
having perpendicularly aligned alkyl chains or fluorocarbon chains,
like lecithin or quaternary ammonium surfactants such as HTAB
(hexadecyl-trimethyl ammonium bromide), DMOAP
(N,N-dimethyl-N-octadecyl-3-aminopropyltrimethoxysilyl chloride) or
N-perfluoroctylsulphonyl-3-aminopropyltrimethyl ammonium iodide
(Uchida et al.), silane polymers like polymethoxysilane or
fluorinated polymers like Teflon.
[0147] Polymerization of the polymerizable mesogenic material takes
place by exposing it to heat or actinic radiation. Actinic
radiation means irradiation with light, like UV light, IR light or
visible light, irradiation with X-rays or gamma rays or irradiation
with high energy particles, such as ions or electrons. Preferably
polymerization is carried out by UV irradiation. As a source for
actinic radiation for example a single UV lamp or a set of UV lamps
can be used. When using a high lamp power the curing time can be
reduced. Another possible source for actinic radiation is a laser,
like e.g. a UV laser, an IR laser or a visible laser.
[0148] The polymerization is preferably carried out in the presence
of an initiator absorbing at the wavelength of the actinic
radiation. For example, when polymerizing by means of UV light, a
photoinitiator can be used that decomposes under UV irradiation to
produce free radicals or ions that start the polymerization
reaction. When curing polymerizable mesogens with acrylate or
methacrylate groups, preferably a radical photoinitiator is used,
when curing polymerizable mesogens vinyl and epoxide groups,
preferably a cationic photoinitiator is used. It is also possible
to use a polymerization initiator that decomposes when heated to
produce free radicals or ions that start the polymerization. As a
photoinitiator for radical polymerization for example the
commercially available Irgacure 651, Irgacure 184, Darocure 1173 or
Darocure 4205 (all from Ciba Geigy AG) can be used, whereas in case
of cationic photopolymerization the commercially available UVI 6974
(Union Carbide) can be used.
[0149] The polymerizable mesogenic material preferably comprises
0.01 to 10%, very preferably 0.05 to 5%, in particular 0.1 to 3% of
a polymerization initiator. UV photoinitiators are preferred, in
particular radicalic UV photoinitiators.
[0150] The curing time is dependening, inter alia, on the
reactivity of the polymerizable mesogenic material, the thickness
of the coated layer, the type of polymerization initiator and the
power of the UV lamp. The curing time according to the invention is
preferably not longer than 10 minutes, particularly preferably not
longer than 5 minutes and very particularly preferably shorter than
2 minutes. For mass production short curing times of 3 minutes or
less, very preferably of 1 minute or less, in particular of 30
seconds or less, are preferred.
[0151] In addition to polymerization initiators the polymerizable
material may also comprise one or more other suitable components
such as, for example, catalysts, sensitizers, stabilizers,
inhibitors, co-reacting monomers, surface-active compounds,
lubricating agents, wetting agents, dispersing agents, hydrophobing
agents, adhesive agents, flow improvers, defoaming agents,
deaerators, diluents, reactive diluents, auxiliaries, colourants,
dyes or pigments.
[0152] In particular the addition of stabilizers is preferred in
order to prevent undesired spontaneous polymerization of the
polymerizable material for example during storage. As stabilizers
in principal all compounds can be used that are known to the
skilled in the art for this purpose. These compounds are
commercially available in a broad variety. Typical examples for
stabilizers are 4-ethoxyphenol or butylated hydroxytoluene
(BHT).
[0153] Other additives, like e.g. chain transfer agents, can also
be added to the polymerizable material in order to modify the
physical properties of the inventive polymer film. When adding a
chain transfer agent, such as monofunctional thiol compounds like
e.g. dodecane thiol or multifunctional thiol compounds like e.g.
trimethylpropane tri(3-mercaptopropionate), to the polymerizable
material, the length of the free polymer chains and/or the length
of the polymer chains between two crosslinks in the inventive
polymer film can be controlled. When the amount of the chain
transfer agent is increased, the polymer chain length in the
obtained polymer film is decreasing.
[0154] It is also possible, in order to increase crosslinking of
the polymers, to add up to 20% of a non mesogenic compound with two
or more polymerizable functional groups to the polymerizable
material alternatively or in addition to the di- or multifunctional
polymerizable mesogenic compounds to increase crosslinking of the
polymer. Typical examples for difunctional non mesogenic monomers
are alkyldiacrylates or alkyldimethacrylates with alkyl groups of 1
to 20 C atoms. Typical examples for non mesogenic monomers with
more than two polymerizable groups are
trimethylpropanetrimethacrylate or pentaerythritoltetraacrylat-
e.
[0155] In another preferred embodiment the mixture of polymerizable
material comprises up to 70%, preferably 3 to 50% of a non
mesogenic compound with one polymerizable functional group. Typical
examples for monofunctional non mesogenic monomers are
alkylacrylates or alkylmethacrylates.
[0156] It is also possible to add, for example, a quantity of up to
20% by weight of a non polymerizable liquid-crystalline compound to
adapt the optical properties of the polymer film.
[0157] In some cases it is of advantage to apply a second substrate
to aid alignment and exclude oxygen that may inhibit the
polymerization. Alternatively the curing can be carried out under
an atmosphere of inert gas. However, curing in air is also possible
using suitable photoinitiators and high UV lamp power. When using a
cationic photoinitiator oxygen exclusion most often is not needed,
but water should be excluded. In a preferred embodiment of the
invention the polymerization of the polymerizable mesogenic
material is carried out under an atmosphere of inert gas,
preferably under a nitrogen atmosphere.
[0158] To obtain a polymer film with the desired molecular
orientation the polymerization has to be carried out in the liquid
crystal phase of the polymerizable mesogenic material. Therefore,
preferably polymerizable mesogenic compounds or mixtures with low
melting points and broad liquid crystal phase ranges are used. The
use of such materials allows to reduce the polymerization
temperature, which makes the polymerization process easier and is a
considerable advantage especially for mass production.
[0159] The selection of suitable polymerization temperatures
depends mainly on the clearing point of the polymerizable material
and inter alia on the softening point of the substrate. Preferably
the polymerization temperature is at least 30 degrees below the
clearing temperature of the polymerizable mesogenic mixture.
[0160] The alignment layer and the method to control the
electrooptical steepness of the present invention are suitable for
liquid crystal displays, in particular those wherein homeotropic
surface alignment is required, such as displays of the VA
(vertically aligned) mode like VAN (vertically aligned nematic) or
VAC (vertically aligned cholesteric), displays of the ECB
(electrically controlled birefringence), DAP (deformation of
aligned phases) or CSH (colour super homeotropic), mode,
cholesteric displays of e.g. the SSCT or PSCT (surface or polymer
stabilized cholesteric texture) mode, or displays of the
flexoelectric or ULH (uniformly lying helix) mode. They can be
applied to displays of the transmissive, reflective or
transflective type. It may also be used in displays with hybrid
alignment, wherein the liquid crystal molecules at one surface of
the LC cell exhibit homeotropic alignment and on the opposite
surface exhibit planar alignment, like for example displays of the
HAN (hybrid aligned nematic) mode.
[0161] A liquid crystal display of the present invention preferably
comprises
[0162] a liquid crystal cell comprising a liquid crystal medium
provided between a first and a second electrode, at least one of
which is light-transmissive, and wherein the liquid crystal
molecules in said medium are oriented homeotropically, i.e.
perpendicular to the electrodes, when no external field is applied,
and
[0163] an alignment layer according to the present invention, which
is provided on the inner surface of at least one of said first and
the second electrode such that it is directly in contact with the
liquid crystal medium and induces homeotropic edge alignment in the
liquid crystal medium,
[0164] and may further comprise
[0165] a first polarizer located on one side of the liquid crystal
cell,
[0166] optionally a second polarizer located such that the liquid
crystal cell is sandwiched between the first and second
polarizer,
[0167] optionally one or more optical compensation or retardation
layers located adjacent to the liquid crystal cell and/or to the
first and second polarizer,
[0168] optionally a backlight,
[0169] and optionally further components.
[0170] Further preferred are bistable or multistable devices, in
particular nematic displays, comprising an alignment layer of the
present invention, in particular an alignment layer comprising a
polymerized smectic A LC material. In these displays the liquid
crystal medium adopts at least two stable configurations. Suitable
bistable or multistable displays where the alignment layer can be
used to induce homeotropic alignment are described for example in
WO98/50821, EP 0 302 479, Martinot-Lagarde et al., Phys. Rev. Left.
(2000), 84(17), 3871-3874, or in G. P. Bryan-Brown, C. V. Brown, J.
C. Jones, E. L. Wood, I. C. Sage, P. Brett and J. Rudin, (1997)
"Grating aligned bistable nematic device" Proceedings of Society
for Information Display International Symposium. Digest of
Technical Papers, Volume XXVIII, Boston, Mass., USA, May 1997,
Chapter 5.3, pp 37-40.
[0171] Further preferred are bistable or multistable display
devices comprising an alignment layer of the present invention, in
particular an alignment layer comprising a polymerized smectic A LC
material, and comprising a liquid crystal medium with a chiral
nematic or cholesteric phase, which can be switched between at
least two different stable states, one of which is a homeotropic
state, for example by application of an electric field. Suitable
examples are SSCT or PSCT displays, as described for example in WO
92/19695, U.S. Pat. No. 5,384,067, U.S. Pat. No. 5,453,863, U.S.
Pat. No. 6,172,720 or U.S. Pat. No. 5,661,533, and displays of the
flexoelectric or ULH mode, as described for example in EP 0 971
016, GB 2 356 629, or Coles, H. J., Musgrave, B., Coles, M. J., and
Willmoft, J., J. Mater. Chem., 11, p. 2709-2716 (2001).
[0172] Apart from liquid crystal displays, the alignment layers of
the present invention can also be used in other liquid crystal
devices such as for example spatial light modulators for optical
computing, or optical switches for routing optical signals.
[0173] Furthermore, the alignment layers of the present invention
can be used as substrates, or as alignment or auxiliary layers
applied on a substrate, in the preparation of anisotropic or liquid
crystal polymer films with homeotropic structure from polymerisable
LC materials, in order to induce homeotropic orientation in the
polymerisable LC material. Anisotropic films with homeotropic
structure are described for example in WO 98/00475 and can be used
for a wide variety of applications, like for example as
compensators or optical retarders.
[0174] The examples below are intended to illustrate the invention
without representing a limitation. Above and below, percentages are
by weight; all temperatures are given in degrees celsius.
[0175] The following abbreviations are used: 8
[0176] In addition:
[0177] .DELTA.n denotes the optical anisotropy measured at
20.degree. C. and 589 nm,
[0178] n.sub.e denotes the extraordinary refractive index at
20.degree. C. and 589 nm,
[0179] .DELTA..epsilon. denotes the dielectric anisotropy at
20.degree. C.,
[0180] .epsilon..parallel. denotes the dielectric constant in the
parallel direction to the molecular axis, and
[0181] cp. denotes the clearing point [.degree. C.].
EXAMPLE 1
[0182] The following polymerizable mixture is formulated
2 Compound (1) 32.67% compound (2) 18.67% compound (3) 21.00%
compound (4) 21.00% Dodecanol 1.02% BHT 0.04% Irgacure 907 5.60% 9
10 11 12
[0183] Compound (1) is described in WO 98/00428. Compound (2) can
be prepared in anaolgy to the methods described in DE 195,04,224.
Compounds (3) and (4) can be prepared in analogy to the methods
described in WO 93/22397. Irgacure 907.RTM. is a photoinitiator
commercially available from Ciba Geigy. BHT (butylated hydroxy
toluene) is a commercially available stabilizer.
[0184] The mixture is coated as a thin layer onto a silica coated
PET substrate and cured at 60.degree. C. by irradiation with UV
light to give a polymer film of 0.1 .mu.m thickness with
homeotropic orientation.
Example 2
[0185] The following polymerizable mixture is formulated
3 Compound (1) 51.90% compound (2) 18.86% compound (3) 23.58%
Irgacure 907 5.66%
[0186] A polymer film with homeotropic orientation is prepared as
described in example 1 and is suitable as homeotropic alignment in
liquid crystal displays.
Example 3
[0187] To demonstrate the influence of the surface anchoring energy
of an alignment layer of the present invention on the electrooptic
properties of a liquid crystal display, computer simulations were
carried out with an Autronic Melchers DIMOS system for a standard
display cell of the vertically aligned (VA mode).
[0188] The VA cell has the following parameters: Two plane-parallel
plates spaced apart at a distance of 4 .mu.m and, on the inside of
the plates, electrode layers covered with a homeotropic alignment
layer of the present invention that can be prepared as described
above. The cell contains the following nematic liquid crystal
medium
4 CCH-34 3.75% cp. 80 CCP-21FF 11.25% .DELTA.n 0.1002 CCP-302FF
13.50% n.sub.e 1.5825 CCP-31FF 11.00% .DELTA..epsilon. -4.4
CCP-502FF 14.00% .epsilon..sub..parallel. 3.8 PCH-301 14.75%
PCH-302FF 13.00% PCH-502FF 12.75% PGIGI-3-F 6.00%
[0189] which is homeotropically aligned when no electric field is
applied, and further contains a chiral dopant with a twist and a
concentration such that a twist angle of 90.degree. is induced in
the medium when an electric field is applied.
[0190] The surface anchoring strength of the liquid crystal
molecules at the cell wall induced by the alignment layer is
expressed by the tilt anchoring parameter .pi.. A value of .pi.=0
means that the liquid crystal molecules are strongly anchored. A
value of .pi.=1 means that the liquid crystal molecules are freely
rotating, without being significantly anchored. The values of .pi.
can be correlated to experimental data according to Mada et al.,
Jap. Journ. Appi. Phys. 38, L1118-L1120 (1999). Thus, a value of
.pi.=1 (not anchored) corresponds to a surface anchoring energy of
about 10.sup.-10 J/m.sup.2, and a value of .pi.=0 (strongly
anchored) corresponds to a surface anchoring energy of about
10.sup.-5 J/m.sup.2.
[0191] FIGS. 1 to 4 show the electrooptic curve (transmission
versus applied voltage) and the full steepness curve (numerical
gradient of the electrooptical curve) of the above VA cell, wherein
the LC medium is in contact with an alignment layer that has a
given tilt anchoring parameter .pi.. The steepness is defined as
V.sub.90/V.sub.10, wherein V.sub.90 and V.sub.10 are the voltages
of the electrooptic curve at 90% and 10% of the maximum
transmission, respectively.
[0192] FIGS. 1.1 and 1.2 show the electrooptic curve and full
steepness curve, respectively, for the VA cell comprising an
alignment layer with a tilt anchoring parameter of 0.
[0193] FIGS. 2.1 and 2.2 show the electrooptic curve and full
steepness curve, respectively, for the VA cell comprising an
alignment layer with a tilt anchoring parameter of 0.4.
[0194] FIGS. 3.1 and 3.2 show the electrooptic curve and full
steepness curve, respectively, for the VA cell comprising an
alignment layer with a tilt anchoring parameter of 1.
[0195] FIGS. 4.1 and 4.2 show the electrooptic curve and full
steepness curve, respectively, for the VA cell comprising an
alignment layer with a tilt anchoring parameter of (from left to
right) 1, 0.8, 0.6, 0.4 and 0, respectively.
[0196] The results clearly demonstrate that there is a strong
correlation between the surface anchoring energy of the vertically
aligned liquid crystal molecules and the corresponding steepness of
the electrooptical curve. The steepness of the electrooptical curve
increases with decreasing surface anchoring energy, i.e. increasing
value of .pi..
[0197] The preceding examples can be repeated with similar success
by substituting the generically or specifically described reactants
and/or operating conditions of this invention for those used in the
preceding examples.
[0198] From the foregoing description, one skilled in the art can
easily ascertain the essential characteristics of this invention,
and without departing from the spirit and scope thereof, can make
various changes and modifications of the invention to adapt it to
various usages and conditions.
* * * * *